This invention concerns manhole covers and more particularly latching systems for manhole covers which may be locked to prevent removal by unauthorized persons but which may selectively be unlocked to allow removal by an authorized person when necessary (see U.S. Pat. No. 6,764,261.
The present inventor has also previously developed manhole latching systems which allow relief of gas pressures developed in a covered vault in case of an explosion in order to prevent damage to the vault while preventing launching of the manhole cover into the air entirely free of the frame, see U.S. Pat. Nos. 7,484,908 and 7,712,995 for a description of such a manhole locking system.
In an explosion of flammable gases, the vault chamber which is covered by a manhole cover, the cover described therein is allowed to rise up off the frame to a limited extent due to a clearance between latching lugs and a frame stop feature which is forcefully contacted by the lugs in an explosion driving the manhole cover up against the stop.
This forcible contact causes severe stress due to the great upward speed of a cover blown up by an explosion in the vault chamber. This stress creates a potential failure of the lug with the possibility that the cover can be blown free of the frame, presenting a safety hazard from the flying cover and the subsequent absence of a cover over the vault chamber.
Another disadvantage of selectively releasable latching systems controlling removal of the cover is the significant costs involved in adding the latching components to manhole covers.
Vault chamber explosions are of varying severity, and it would be advantageous to minimize the height the cover moves up in safely dissipating the explosion induced stresses. That is, it would reduce the hazards to passing traffic if the height the cover reaches for an explosion of lesser severity were reduced.
Flooding of the chamber with water can similarly create a need for allowing the cover to rise up off its seat in the frame while preventing the manhole cover from being washed completely free of the frame.
It is an object of the present invention to provide a manhole cover latching system which in an explosion allows the cover to rise up off its seat in the frame but cushions the impact of the lugs with the aligned stops to minimize the chances of lug breakage and consequent escape of the manhole cover from the frame.
It is a further object of the present invention to provide a low cost selectively operated latching system.
It is also an object of the invention to provide a manhole latching system which can relieve pressure caused by flooding or explosion events in the covered vault chamber while minimizing the height to which the manhole cover rises while dissipating the pressures developed in the vault chamber from such events.